The gut-brain vagal axis scales hippocampal memory processes and plasticity.

The vagus nerve serves as an interoceptive relay between the body and the brain. Despite its well-established role in feeding behaviors, energy metabolism, and cognitive functions, the intricate functional processes linking the vagus nerve to the hippocampus and its contribution to learning and memory dynamics remain still elusive. Here, we investigated whether and how the gut-brain vagal axis contributes to hippocampal learning and memory processes at behavioral, functional, cellular, and molecular levels. Our results indicate that the integrity of the vagal axis is essential for long-term recognition memories, while sparing other forms of memory. In addition, by combing multi-scale approaches, our findings show that the gut-brain vagal tone exerts a permissive role in scaling intracellular signaling events, gene expressions, hippocampal dendritic spines density as well as functional long-term plasticities (LTD and LTP). These results highlight the critical role of the gut-brain vagal axis in maintaining the spontaneous and homeostatic functions of hippocampal ensembles and in regulating their learning and memory functions. In conclusion, our study provides comprehensive insights into the multifaceted involvement of the gut-brain vagal axis in shaping time-dependent hippocampal learning and memory dynamics. Understanding the mechanisms underlying this interoceptive body-brain neuronal communication may pave the way for novel therapeutic approaches in conditions associated with cognitive decline, including neurodegenerative disorders.

Dendritic Spines in Learning and Memory: From First Discoveries to Current Insights.

The central nervous system is composed of neural ensembles, and their activity patterns are neural correlates of cognitive functions. Those ensembles are networks of neurons connected to each other by synapses. Most neurons integrate synaptic signal through a remarkable subcellular structure called spine. Dendritic spines are protrusions whose diverse shapes make them appear as a specific neuronal compartment, and they have been the focus of studies for more than a century. Soon after their first description by Ramón y Cajal, it has been hypothesized that spine morphological changes could modify neuronal connectivity and sustain cognitive abilities. Later studies demonstrated that changes in spine density and morphology occurred in experience-dependent plasticity during development, and in clinical cases of mental retardation. This gave ground for the assumption that dendritic spines are the particular locus of cerebral plasticity. With the discovery of synaptic long-term potentiation, a research program emerged with the aim to establish whether dendritic spine plasticity could explain learning and memory. The development of live imaging methods revealed on the one hand that dendritic spine remodeling is compatible with learning process and, on the other hand, that their long-term stability is compatible with lifelong memories. Furthermore, the study of the mechanisms of spine growth and maintenance shed new light on the rules of plasticity. In behavioral paradigms of memory, spine formation or elimination and morphological changes were found to correlate with learning. In a last critical step, recent experiments have provided evidence that dendritic spines play a causal role in learning and memory.

Serotonin sensing by microglia conditions the proper development of neuronal circuits and of social and adaptive skills.

The proper maturation of emotional and sensory circuits requires fine-tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors in mice. Since the main 5-HT sensor in microglia is the 5-HT2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditional invalidation of the Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during early postnatal development affects the phagolysosomal compartment of these cells and their proximity to dendritic spines and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.

A positive allosteric modulator of mGlu4 receptors restores striatal plasticity in an animal model of l-Dopa-induced dyskinesia.

By decreasing glutamate transmission, mGlu4 receptor positive allosteric modulators (mGlu4-PAM), in combination with levodopa (l-DOPA) may restore the synergy between glutamatergic and dopaminergic transmissions, thus maximizing the improvement of motor function in Parkinson's disease (PD). This study aimed to clarify the effects of foliglurax, a selective mGlu4-PAM, on the loss of bidirectional synaptic plasticity associated with l-DOPA-induced dyskinesia (LID). Behavioral assessments compared dyskinesia intensity in 6-hydroxydopamine (6-OHDA)-lesioned rats treated with l-DOPA or l-DOPA plus foliglurax. In slices from the same rats, patch-clamp techniques were used to examine electrophysiological differences in glutamatergic synapses, evaluating the EPSCs mediated by NMDA and AMPA receptors in striatal spiny projection neurons. High-frequency stimulation of corticostriatal fibers was used as long-term potentiation (LTP)-inducing protocol. Conversely, 15 min of low-frequency stimulation was applied to depotentiate LTP. The density of dendritic spines was measured in striatal slices in the same experimental conditions. Our results show that, in corticostriatal slices, foliglurax decreased spontaneous glutamatergic transmission in both sham-operated and 6-OHDA lesioned rats. When co-administered with l-DOPA in 6-OHDA-lesioned rats, foliglurax fully restored dendritic spine density in a dose-dependent manner. Moreover, this co-treatment rescued striatal bidirectional plasticity and attenuated the intensity of l-DOPA-induced dyskinesia. This is the first demonstration in an animal model of PD and dyskinesia that a mGlu4 PAM can restore striatal synaptic plasticity.

Cell-Type-Specific Adaptions in Striatal Medium-Sized Spiny Neurons and Their Roles in Behavioral Responses to Drugs of Abuse.

Drug addiction is defined as a compulsive pattern of drug-seeking- and taking- behavior, with recurrent episodes of abstinence and relapse, and a loss of control despite negative consequences. Addictive drugs promote reinforcement by increasing dopamine in the mesocorticolimbic system, which alters excitatory glutamate transmission within the reward circuitry, thereby hijacking reward processing. Within the reward circuitry, the striatum is a key target structure of drugs of abuse since it is at the crossroad of converging glutamate inputs from limbic, thalamic and cortical regions, encoding components of drug-associated stimuli and environment, and dopamine that mediates reward prediction error and incentive values. These signals are integrated by medium-sized spiny neurons (MSN), which receive glutamate and dopamine axons converging onto their dendritic spines. MSN primarily form two mostly distinct populations based on the expression of either DA-D1 (D1R) or DA-D2 (D2R) receptors. While a classical view is that the two MSN populations act in parallel, playing antagonistic functional roles, the picture seems much more complex. Herein, we review recent studies, based on the use of cell-type-specific manipulations, demonstrating that dopamine differentially modulates dendritic spine density and synapse formation, as well as glutamate transmission, at specific inputs projecting onto D1R-MSN and D2R-MSN to shape persistent pathological behavioral in response to drugs of abuse. We also discuss the identification of distinct molecular events underlying the detrimental interplay between dopamine and glutamate signaling in D1R-MSN and D2R-MSN and highlight the relevance of such cell-type-specific molecular studies for the development of innovative strategies with potential therapeutic value for addiction. Because drug addiction is highly prevalent in patients with other psychiatric disorders when compared to the general population, we last discuss the hypothesis that shared cellular and molecular adaptations within common circuits could explain the co-occurrence of addiction and depression. We will therefore conclude this review by examining how the nucleus accumbens (NAc) could constitute a key interface between addiction and depression.

Disrupting D1-NMDA or D2-NMDA receptor heteromerization prevents cocaine's rewarding effects but preserves natural reward processing.

Addictive drugs increase dopamine in the nucleus accumbens (NAc), where it persistently shapes excitatory glutamate transmission and hijacks natural reward processing. Here, we provide evidence, from mice to humans, that an underlying mechanism relies on drug-evoked heteromerization of glutamate N-methyl-d-aspartate receptors (NMDAR) with dopamine receptor 1 (D1R) or 2 (D2R). Using temporally controlled inhibition of D1R-NMDAR heteromerization, we unraveled their selective implication in early phases of cocaine-mediated synaptic, morphological, and behavioral responses. In contrast, preventing D2R-NMDAR heteromerization blocked the persistence of these adaptations. Interfering with these heteromers spared natural reward processing. Notably, we established that D2R-NMDAR complexes exist in human samples and showed that, despite a decreased D2R protein expression in the NAc, individuals with psychostimulant use disorder display a higher proportion of D2R forming heteromers with NMDAR. These findings contribute to a better understanding of molecular mechanisms underlying addiction and uncover D2R-NMDAR heteromers as targets with potential therapeutic value.

Npas4 regulates medium spiny neuron physiology and gates cocaine-induced hyperlocomotion.

We show here that the transcription factor Npas4 is an important regulator of medium spiny neuron spine density and electrophysiological parameters and that it determines the magnitude of cocaine-induced hyperlocomotion in mice. Npas4 is induced by synaptic stimuli that cause calcium influx, but not dopaminergic or PKA-stimulating input, in mouse medium spiny neurons and human iPSC-derived forebrain organoids. This induction is independent of ubiquitous kinase pathways such as PKA and MAPK cascades, and instead depends on calcineurin and nuclear calcium signalling. Npas4 controls a large regulon containing transcripts for synaptic molecules, such as NMDA receptors and VDCC subunits, and determines in vivo MSN spine density, firing rate, I/O gain function and paired-pulse facilitation. These functions at the molecular and cellular levels control the locomotor response to drugs of abuse, as Npas4 knockdown in the nucleus accumbens decreases hyperlocomotion in response to cocaine in male mice while leaving basal locomotor behaviour unchanged.

Loss of floor plate Netrin-1 impairs midline crossing of corticospinal axons and leads to mirror movements.

In humans, execution of unimanual movements requires lateralized activation of the primary motor cortex, which then transmits the motor command to the contralateral hand through the crossed corticospinal tract (CST). Mutations in NTN1 alter motor control lateralization, leading to congenital mirror movements. To address the role of midline Netrin-1 on CST development and subsequent motor control, we analyze the morphological and functional consequences of floor plate Netrin-1 depletion in conditional knockout mice. We show that depletion of floor plate Netrin-1 in the brainstem critically disrupts CST midline crossing, whereas the other commissural systems are preserved. The only associated defect is an abnormal entry of CST axons within the inferior olive. Alteration of CST midline crossing results in functional ipsilateral projections and is associated with abnormal symmetric movements. Our study reveals the role of Netrin-1 in CST development and describes a mouse model recapitulating the characteristics of human congenital mirror movements.

CYP46A1 gene therapy deciphers the role of brain cholesterol metabolism in Huntington's disease.

Dysfunctions in brain cholesterol homeostasis have been extensively related to brain disorders. The main pathway for brain cholesterol elimination is its hydroxylation into 24S-hydroxycholesterol by the cholesterol 24-hydrolase, CYP46A1. Increasing evidence suggests that CYP46A1 has a role in the pathogenesis and progression of neurodegenerative disorders, and that increasing its levels in the brain is neuroprotective. However, the mechanisms underlying this neuroprotection remain to be fully understood. Huntington's disease is a fatal autosomal dominant neurodegenerative disease caused by an abnormal CAG expansion in huntingtin's gene. Among the multiple cellular and molecular dysfunctions caused by this mutation, altered brain cholesterol homeostasis has been described in patients and animal models as a critical event in Huntington's disease. Here, we demonstrate that a gene therapy approach based on the delivery of CYP46A1, the rate-limiting enzyme for cholesterol degradation in the brain, has a long-lasting neuroprotective effect in Huntington's disease and counteracts multiple detrimental effects of the mutated huntingtin. In zQ175 Huntington's disease knock-in mice, CYP46A1 prevented neuronal dysfunctions and restored cholesterol homeostasis. These events were associated to a specific striatal transcriptomic signature that compensates for multiple mHTT-induced dysfunctions. We thus explored the mechanisms for these compensations and showed an improvement of synaptic activity and connectivity along with the stimulation of the proteasome and autophagy machineries, which participate to the clearance of mutant huntingtin (mHTT) aggregates. Furthermore, BDNF vesicle axonal transport and TrkB endosome trafficking were restored in a cellular model of Huntington's disease. These results highlight the large-scale beneficial effect of restoring cholesterol homeostasis in neurodegenerative diseases and give new opportunities for developing innovative disease-modifying strategies in Huntington's disease.

Serotonergic Signaling Controls Input-Specific Synaptic Plasticity at Striatal Circuits.

Monoaminergic modulation of cortical and thalamic inputs to the dorsal striatum (DS) is crucial for reward-based learning and action control. While dopamine has been extensively investigated in this context, the synaptic effects of serotonin (5-HT) have been largely unexplored. Here, we investigated how serotonergic signaling affects associative plasticity at glutamatergic synapses on the striatal projection neurons of the direct pathway (dSPNs). Combining chemogenetic and optogenetic approaches reveals that impeding serotonergic signaling preferentially gates spike-timing-dependent long-term depression (t-LTD) at thalamostriatal synapses. This t-LTD requires dampened activity of the 5-HT4 receptor subtype, which we demonstrate controls dendritic Ca2+ signals by regulating BK channel activity, and which preferentially localizes at the dendritic shaft. The synaptic effects of 5-HT signaling at thalamostriatal inputs provide insights into how changes in serotonergic levels associated with behavioral states or pathology affect striatal-dependent processes.

Cocaine increases dopaminergic connectivity in the nucleus accumbens.

The development of addictive behavior is associated with functional and structural plasticity in the mesocorticolimbic pathway. Increased connectivity upon cocaine administration has been inferred from increases in dendritic spine density, but without observations of presynaptic elements. Recently, we established a method that enables analyses of both dendritic spines and glutamatergic boutons and presented evidence that cocaine induces changes in striatal connectivity. As the pharmacological and behavioral effects of cocaine directly implicate dopaminergic neurons and their afferents, a remaining question is whether dopaminergic striatal innervations also undergo structural plasticity. To address this issue, we generated transgenic mice in which the fluorophore tdTomato is expressed under the promoter of the dopamine transporter gene. In these mice, specific labeling of dopaminergic boutons was observed in the striatum. Of note, the accordance of our results for control mice with previous electron microscopy studies confirms that our method can be used to decipher the spatial organization of boutons in relation to dendritic elements. Following repeated cocaine administration that led to behavioral locomotor sensitization, an increased density of dopaminergic boutons was observed 1 day later in the nucleus accumbens shell specifically, and not in other striatal regions. Combined labeling of dopaminergic boutons and striatal dendrites showed that cocaine significantly increased the percentage of dendritic spines associated with a dopaminergic bouton. Our results show that chronic cocaine administration induces structural plasticity of dopaminergic boutons that could participate in dopamine-dependent neuronal adaptations in the striatum.

Rapid Synaptogenesis in the Nucleus Accumbens Is Induced by a Single Cocaine Administration and Stabilized by Mitogen-Activated Protein Kinase Interacting Kinase-1 Activity.

BACKGROUND: Repeated cocaine exposure produces new spine formation in striatal projection neurons (SPNs) of the nucleus accumbens. However, an acute exposure to cocaine can trigger long-lasting synaptic plasticity in SPNs leading to behavioral alterations. This raises the intriguing question as to whether a single administration of cocaine could enduringly modify striatal connectivity. METHODS: A three-dimensional morphometric analysis of presynaptic glutamatergic boutons and dendritic spines was performed on SPNs 1 hour and 1 week after a single cocaine administration. Time-lapse two-photon microscopy in adult slices was used to determine the precise molecular-events sequence responsible for the rapid spine formation. RESULTS: A single injection triggered a rapid synaptogenesis and persistent increase in glutamatergic connectivity in SPNs from the shell part of the nucleus accumbens, specifically. Synapse formation occurred through clustered growth of active spines contacting pre-existing axonal boutons. Spine growth required extracellular signal-regulated kinase activation, while spine stabilization involved transcription-independent protein synthesis driven by mitogen-activated protein kinase interacting kinase-1, downstream from extracellular signal-regulated kinase. The maintenance of new spines driven by mitogen-activated protein kinase interacting kinase-1 was essential for long-term connectivity changes induced by cocaine in vivo. CONCLUSIONS: Our study originally demonstrates that a single administration of cocaine is able to induce stable synaptic rewiring in the nucleus accumbens, which will likely influence responses to subsequent drug exposure. It also unravels a new functional role for cocaine-induced extracellular signal-regulated kinase pathway independently of nuclear targets. Finally, it reveals that mitogen-activated protein kinase interacting kinase-1 has a pivotal role in cocaine-induced connectivity.

DiAna, an ImageJ tool for object-based 3D co-localization and distance analysis.

We present a new plugin for ImageJ called DiAna, for Distance Analysis, which comes with a user-friendly interface. DiAna proposes robust and accurate 3D segmentation for object extraction. The plugin performs automated object-based co-localization and distance analysis. DiAna offers an in-depth analysis of co-localization between objects and retrieves 3D measurements including co-localizing volumes and surfaces of contact. It also computes the distribution of distances between objects in 3D. With DiAna, we furthermore introduce an original method, which allows for estimating the statistical significance of object co-localization. DiAna offers a complete and intuitive 3D image analysis tool for biologists.

CYP46A1, the rate-limiting enzyme for cholesterol degradation, is neuroprotective in Huntington's disease.

Huntington's disease is an autosomal dominant neurodegenerative disease caused by abnormal polyglutamine expansion in huntingtin (Exp-HTT) leading to degeneration of striatal neurons. Altered brain cholesterol homeostasis has been implicated in Huntington's disease, with increased accumulation of cholesterol in striatal neurons yet reduced levels of cholesterol metabolic precursors. To elucidate these two seemingly opposing dysregulations, we investigated the expression of cholesterol 24-hydroxylase (CYP46A1), the neuronal-specific and rate-limiting enzyme for cholesterol conversion to 24S-hydroxycholesterol (24S-OHC). CYP46A1 protein levels were decreased in the putamen, but not cerebral cortex samples, of post-mortem Huntington's disease patients when compared to controls. Cyp46A1 mRNA and CYP46A1 protein levels were also decreased in the striatum of the R6/2 Huntington's disease mouse model and in SThdhQ111 cell lines. In vivo, in a wild-type context, knocking down CYP46A1 expression in the striatum, via an adeno-associated virus-mediated delivery of selective shCYP46A1, reproduced the Huntington's disease phenotype, with spontaneous striatal neuron degeneration and motor deficits, as assessed by rotarod. In vitro, CYP46A1 restoration protected SThdhQ111 and Exp-HTT-expressing striatal neurons in culture from cell death. In the R6/2 Huntington's disease mouse model, adeno-associated virus-mediated delivery of CYP46A1 into the striatum decreased neuronal atrophy, decreased the number, intensity level and size of Exp-HTT aggregates and improved motor deficits, as assessed by rotarod and clasping behavioural tests. Adeno-associated virus-CYP46A1 infection in R6/2 mice also restored levels of cholesterol and lanosterol and increased levels of desmosterol. In vitro, lanosterol and desmosterol were found to protect striatal neurons expressing Exp-HTT from death. We conclude that restoring CYP46A1 activity in the striatum promises a new therapeutic approach in Huntington's disease.

Improving axial resolution in confocal microscopy with new high refractive index mounting media.

Resolution, high signal intensity and elevated signal to noise ratio (SNR) are key issues for biologists who aim at studying the localisation of biological structures at the cellular and subcellular levels using confocal microscopy. The resolution required to separate sub-cellular biological structures is often near to the resolving power of the microscope. When optimally used, confocal microscopes may reach resolutions of 180 nm laterally and 500 nm axially, however, axial resolution in depth is often impaired by spherical aberration that may occur due to refractive index mismatches. Spherical aberration results in broadening of the point-spread function (PSF), a decrease in peak signal intensity when imaging in depth and a focal shift that leads to the distortion of the image along the z-axis and thus in a scaling error. In this study, we use the novel mounting medium CFM3 (Citifluor Ltd., UK) with a refractive index of 1.518 to minimize the effects of spherical aberration. This mounting medium is compatible with most common fluorochromes and fluorescent proteins. We compare its performance with established mounting media, harbouring refractive indices below 1.500, by estimating lateral and axial resolution with sub-resolution fluorescent beads. We show furthermore that the use of the high refractive index media renders the tissue transparent and improves considerably the axial resolution and imaging depth in immuno-labelled or fluorescent protein labelled fixed mouse brain tissue. We thus propose to use those novel high refractive index mounting media, whenever optimal axial resolution is required.

A new automated 3D detection of synaptic contacts reveals the formation of cortico-striatal synapses upon cocaine treatment in vivo.

Addiction can be considered as a form of neuronal adaptation within the reward circuitry. Upon psychostimulant administration, long-term behavioral adaptations are associated with synaptic plasticity and morphological changes of medium spiny neurons (MSN) from the striatum. Increased spine density onto MSN in response to chronic cocaine exposure in mice has been described for more than a decade, but no evidence indicates that these newly formed spines establish connections. We developed a method for labeling, automated detection and morphological analysis of synaptic contacts. Individual labeling of neurons in mice that express the Vesicular GLUtamate Transporter-1 fused to Venus allows visualization of both dendritic spines and axonal boutons. Automated three-dimensional segmentation and morphometric analysis retrieve information on thousands of synapses at high resolution. We used this method to demonstrate that new cortico-striatal connections are formed in the striatum upon chronic cocaine. We also show that the cortical input weight is preserved over other cerebral inputs and that the newly formed spines contact pre-existing axonal boutons. Our results pave the way for other studies, since our method can be applied to any other neuronal type as demonstrated herein for glutamatergic connections on pyramidal neurons and Purkinje cells.

Mature Purkinje cells require the retinoic acid-related orphan receptor-α (RORα) to maintain climbing fiber mono-innervation and other adult characteristics.

Neuronal maturation during development is a multistep process regulated by transcription factors. The transcription factor RORα (retinoic acid-related orphan receptor α) is necessary for early Purkinje cell (PC) maturation but is also expressed throughout adulthood. To identify the role of RORα in mature PCs, we used Cre-lox mouse genetic tools in vivo that delete it specifically from PCs between postnatal days 10-21. Up to 14 d of age, differences between mutant and control PCs were not detectable: both were mono-innervated by climbing fibers (CFs) extending along their well-developed dendrites with spiny branchlets. By week 4, mutant mice were ataxic, some PCs had died, and remaining PC soma and dendrites were atrophic, with almost complete disappearance of spiny branchlets. The innervation pattern of surviving RORα-deleted PCs was abnormal with several immature characteristics. Notably, multiple functional CF innervation was reestablished on these mature PCs, simultaneously with the relocation of CF contacts to the PC soma and their stem dendrite. This morphological modification of CF contacts could be induced even later, using lentivirus-mediated depletion of rora from adult PCs. These data show that the late postnatal expression of RORα cell-autonomously regulates the maintenance of PC dendritic complexity, and the CF innervation status of the PC (dendritic vs somatic contacts, and mono-innervation vs multi-innervation). Thus, the differentiation state of adult neurons is under the control of transcription factors; and in their absence, adult neurons lose their mature characteristics and acquire some characteristics of an earlier developmental stage.

A deconvolution method to improve automated 3D-analysis of dendritic spines: application to a mouse model of Huntington's disease.

Dendritic spines are postsynaptic structures the morphology of which correlates with the strength of synaptic efficacy. Measurements of spine density and spine morphology are achievable using recent imaging and bioinformatics tools. The three-dimensional automated analysis requires optimization of image acquisition and treatment. Here, we studied the critical steps for optimal confocal microscopy imaging of dendritic spines. We characterize the deconvolution process and show that it improves spine morphology analysis. With this method, images of dendritic spines from medium spiny neurons are automatically detected by the software Neuronstudio, which retrieves spine density as well as spine diameter and volume. This approach is illustrated with three-dimensional analysis of dendritic spines in a mouse model of Huntington's disease: the transgenic R6/2 mice. In symptomatic mutant mice, we confirm the decrease in spine density, and the method brings further information and show a decrease in spine volume and dendrite diameter. Moreover, we show a significant decrease in spine density at presymptomatic age which so far has gone unnoticed.

Alterations of molecular and behavioral responses to cocaine by selective inhibition of Elk-1 phosphorylation.

Activation of the extracellular signal-regulated kinase (ERK) signaling pathway in the striatum is crucial for molecular adaptations and long-term behavioral alterations induced by cocaine. In response to cocaine, ERK controls the phosphorylation levels of both mitogen and stress-activated protein kinase 1 (MSK-1), a nuclear kinase involved in histone H3 (Ser10) and cAMP response element binding protein phosphorylation, and Elk-1, a transcription factor involved in serum response element (SRE)-driven gene regulations. We recently characterized the phenotype of msk-1 knock-out mice in response to cocaine. Herein, we wanted to address the role of Elk-1 phosphorylation in cocaine-induced molecular, morphological, and behavioral responses. We used a cell-penetrating peptide, named TAT-DEF-Elk-1 (TDE), which corresponds to the DEF docking domain of Elk-1 toward ERK and inhibits Elk-1 phosphorylation induced by ERKs without modifying ERK or MSK-1 in vitro. The peptide was injected in vivo before cocaine administration in mice. Immunocytochemical, molecular, morphological, and behavioral studies were performed. The TDE inhibited Elk-1 and H3 (Ser10) phosphorylation induced by cocaine, sparing ERK and MSK-1 activation. Consequently, TDE altered cocaine-induced regulation of genes bearing SRE site(s) in their promoters, including c-fos, zif268, ΔFosB, and arc/arg3.1 (activity-regulated cytoskeleton-associated protein). In a chronic cocaine administration paradigm, TDE reversed cocaine-induced increase in dendritic spine density. Finally, the TDE delayed the establishment of cocaine-induced psychomotor sensitization and conditioned-place preference. We conclude that Elk-1 phosphorylation downstream from ERK is a key molecular event involved in long-term neuronal and behavioral adaptations to cocaine.